According to a report on CVG, Overkill Entertainment - developers of multiplayer shooter Payday:…
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"We're trying to be a little coy and a little fun because we want the communities to discover it," Valve' Chet Falisek told PCGamesN, "but those guys are still making Payday: The Heist, we're still making Left 4 Dead."

Specifically, he says the collaboration "is kind of akin to how in TF2 you might see some other games' weapons get translated and put into that game. We're doing something similar to that, but it's a little bit more involved."